Fix typo in Hazy2

While updating the Hazy covers, a string '-2' had been copied from an
online source.  That was not recognized on Mac, and led to failed Hazy
compilations.  Replace that minus with an ASCII equivalent.

docs/latex/hazy2/hazy2.tex

@@ -63,7 +63,7 @@ and test suite
 {\small
 {\em Cover image:}
 Continuum opacities plotted as a function of wavelength for a cloud with
-$\rm{N_H} = 10^{23} \, \rm{cm}^{−2}$.
+$\rm{N_H} = 10^{23} \, \rm{cm}^{-2}$.
 Figure 1 from \citet{Chakraborty2022}.
 }
 \clearpage